Why is it necessary to have both substantive and procedural due process?

Answer 1

Because fairness is not enough; the appearance of fairness is also important.
Anybody who examines the case later needs to be able to reconstruct exactly what happened during it.
Two friends can privately have fairness; if I break your vase, I can simply pay you the value of it in cash, and we're both satisfied. But when things come to court, it is because there is a dispute about fairness, and there has to be a clear record of how the case was decided.